1. Field of the Invention
This invention relates to positive-working photoresist compositions comprising a soluble resin, a photosensitizer, a solvent, and a dye which is soluble in both the photoresist casting solvent and also in aqueous alkaline developer. This invention further relates to a patterning method in which the noted positive-working photoresist compositions are employed. This invention is useful for achieving an improved scum-free photoresist profile shape on reflective semiconductor substrates with an improved photospeed.
2. Photoresist Technology and the Need for This Invention
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473 and 4,409,314 and European Patent Application No. 0092444. These include alkali-soluble phenolic-formaldehyde novolac resins together with light-sensitive materials. Examples of the light sensitive materials are diazoquinones (DAQs) such as the sulfonate and carboxylate esters and the sulfon- and carbonamides obtained by reacting, respectively, o-naphthoquinone diazide sulfonyl and carbonyl halides with hydroxy, polyhydroxy, amino and polyamino ballast compounds (See U.S. Pat. application No. 174,556 filed on July 18, 1950 by Maximillian Paul Schmidt and now abandoned, and U.S. Pat. Nos. 3,046,110, 3,046,122 and 3,046123). The resins and sensitizers are dissolved in an organic casting solvent or mixture of casting solvents and are applied as a dried thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the admixed naphthoquinone sensitizer acts as a dissolution inhibitor with respect to the resin. Upon exposure of selected areas of a coated substrate to actinic radiation, the sensitizer undergoes a radiation induced chemical transformation, and the exposed areas of the coating are rendered more soluble than the unexposed area. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution, while the unexposed areas are largely unaffected. This produces a positive relief resist pattern on the substrate. In most instances, the imagewise-exposed and developed resist pattern resulting on the substrate will be subjected to treatment by a substrate-etchant process. The photoresist pattern on the substrate protects the resist coated areas o: the substrate from the etchant, and thus the etchant is only able to etch the remaining uncoated areas of the substrate which, in the case of a positive photoresist, correspond to the areas previously exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create the latent images in the resist prior to development. The relief pattern of photoresist on the substrate produced by the method just described is useful for various applications, including the manufacture of miniaturized integrated electronic circuit components.
The term PAC as used in this present invention refers to the photoactive component of the resist composition. The PAC generally is sensitive to energetic forms of radiation such as ultraviolet (UV) light, undergoing radiation-induced chemical transformations upon exposure to such radiation.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast and resist resolution capability and resist sidewall angle or wall profile, and resist adhesion. Increased photospeed is important for a photoresist, particularly in applications where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and monochromatic filters. Increased photospeed is also particularly important in applications where a number of exposures are needed, for example in generating multiple patterns in a step-and-repeat process. Increased photospeed is also important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate.
Resist resolution refers to the capability of a resist system to reproduce with a given phototool the smallest multiple equal line/space features of a mask which is utilized during exposure with a high degree of image edge acuity in the developed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very narrow lines and spaces. The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of very large scale integrated (VLSI) electronic circuits on silicon chips. Circuit density on such a chip can only be increased, assuming lithographic techniques are utilized, by increasing the resolution capabilities of the resist.
Another extremely important concern is the ability to maintain constant linewidths, feature sizes, feature shapes and sufficient thickness when positive photoresist is patterned on reflective rough semiconductor substrates or over reflective uneven topography. In this situation, light is reflected at an angle from the uneven areas of the substrate into the photoresist causing localized regions of the photoresist to receive an additional dose of light, which produces an undesirable profile shape and poor edge acuity after development. The loss of profile shape can severely harm the ability to successfully survive subsequent processing steps, such as etching of the underlying substrates. Uneven edge acuity can have highly deleterious effects on the electrical properties of the manufactured semiconductor device.
Also important is the ability to pattern the positive photoresist without leaving undesirable residual deposits, called scum, on the exposed semiconductor substrates. These scum deposits occur in some compositions of the prior art due to insolubility and incompatability of the resist components in the aqueous alkaline developer. This problem can be particularly severe with compositions of the prior art developed in aqueous metal-ion-free developers, such as aqueous tetramethylammoniumhydroxide solutions. Scum deposits can adversely affect the electrical characteristics of the device and the ability to complete further processing steps.
The positive photoresist compositions and processes provided in this invention dramatically improve performance properties on reflective semiconductor substrates while maintaining adequate photospeed. Photoresist absorbance in these compositions is increased by the addition of a bis(phenylazo)resorcinol dye which absorbs at the actinic exposure wavelength. The bis(phenylazo)resorcinol dye functions to absorb stray reflections and thus diminish undesirable profile shapes, giving excellent linewidth control and unexposed film thickness retention. The bis(phenylazo)resorcinol dye enhances the dissolution rate of the exposed photoresist in developer, thus improving photospeed relative to other dyed compositions in the prior art. Finally, the bis(phenylazo)resorcinol dye is compatable with, and soluble in, metal-ion-free aqueous alkaline developers, thus eliminating problems with undesirable scum formation on the semiconductor substrate.
3. Description of the Prior Art
In the J. Electrochemical Society: Solid State Science and Technology vol. 133, pp.192-196 (1986), T. R. Pampalone and F. A. Kuyan describe the effect of the addition o: Macrolex 6G dye to positive photoresist compositions. They found that the required exposure dose was increased by 220 to 520%. The sidewall angle of this composition was rather poor, about 60.degree.-70.degree. , indicating only moderate resolution. The dye, resins, PACs and solvents described in this publication are different from the present invention. It will be shown in the Examples that Macrolex 6G dye is not significantly soluble in aqueous alkaline developer; thus the dissolution rate was decreased, contributing to the increase in the required exposure dose.
In Solid State Technology, pp. 125-130, (Jan., 1988), C. A. Mack described absorption properties of two dyed photoresist compositions in an article titled "Dispelling the Myths About Dyed Photoresist". The dyes he described were Coumarin 314 and Macrolex 6G. Neither dye is soluble in developer and both are different from the present invention. The dyes, resins, PACs and solvents described in this publication are different from the present invention.
His analysis reported that dyed resist compositions may not significantly improve lithographic processing. We will show in the Examples that the dye of our invention does indeed significantly improve resist processing on reflective substrates.
U.S. Pat. No. 4,626,492 (Dec. 2 1986) claims a photoresist composition containing 10-20 percent of trihydroxybenzophenone and 0.1 to 3 percent dye, together with sensitizer and a specific novolac resin composition. The dyes claimed include anthroquinones, coumarins, diphenylmethanes, triphenylmethanes, phenanzines, oxazines, xanthenes and phenylazo(4-dialkylamino)benzene. The function of the trihydroxybenzophenone is to increase the dissolution rate of this composition, thus enhancing the light sensitivity of the photoresist. However, trihydroxybenzophenone additive has the significant disadvantage of being an invitro mutagen (TSCA Report 8E HQ-0484-0510). The dyes, resins, PACs and solvents described in this publication are different from, and inferior in performance compared to the present invention.
Japanese Pat. No. 59,142,538 (8/15/84) claims photoresist compositions containing selected phenyl- and napthyl- azo compounds, including (4-phenylazo)resorcinol. However, it will be shown in this invention that the (4-phenylazo)resorcinol component has insufficient absorbance at 436 nm to be useful as the only dye compound in the composition. The dyes, resins, PACs and solvents described in this publication are different from the present invention.
The dyed positive photoresist compositions of the present invention are distinctly different from the photoresist compositions of the prior art for the following reasons: (1) The use of bis(phenylazo)resorcinol dye compounds in a photoresist composition is new; (2) The combination of chemical components within the composition, namely the dye, resins and solvents, are new.
4. Improvements Offered by This Invention
The following combination of properties of the novel dyed positive photoresist compositions of the present invention are distinctly better than the photoresist compositions of the prior art: (1) The novel bisazo phenolic dye is soluble in aqueous alkaline developer and does not significantly inhibit the dissolution rate of the exposed photoresist film in aqueous alkaline developers; (2) The dye does not significantly harm deep-UV curing processes of the photoresist film; (3) The dye has a strong absorbance over the full range of 365 nm through 436 nm and is useful in a photoresist composition on a variety of different exposure units using 365 nm through 436 nm radiation; (4) The dyed photoresist compositions of the present invention do not cause scumming of the semiconductor substrate upon development of the exposed film; (5) The dyed photoresist compositions have substantially better resolution; (6) The dyed photoresist compositions require less exposure energy than prior dyed photoresist compositions; (7) The novel dye is non-volatile and is not readily sublimed from the resist film and (8) The novel dyed photoresist compositions are stable for at least one year without forming precipitates.